The present invention relates generally to high frequency communication circuits and, more specifically, to maintaining desired circuit component attributes, such as monolithic microwave integrated circuit impedance, through sealing techniques.
With the advancement of new technologies and the opening up of additional radio frequency spectrum for commercial use, the use of wireless communications has flourished in recent years. Accordingly, communication systems providing large amounts of voice and/or data communication have begun to rely more and more heavily upon wireless links. For example, metropolitan communication networks often rely upon wireless links to provide connections where infrastructure, such as fiber optic links, is unavailable or not economical to deploy. Accordingly, point-to-point and point-to-multipoint microwave radio communication links are becoming very common to provide reliable high bandwidth communications.
The demand for such radio communication links has driven the need for reliable and low cost wireless systems and components. Moreover, as the demand for such links has grown the need to be able to economically construct and deploy suitable systems and components has magnified. For example, the use of miniaturization, such as very large scale integrated (VLSI) circuits, surface mount technology, and application specific integrated circuits (ASIC), in constructing communication systems is common both to provide low-cost parts and provide assembly economies as well as to provide a smaller and more energy efficient component which is easier to deploy and operate.
In providing microwave communications equipment, the use of microwave integrated circuits (MIC), particularly monolithic microwave integrated circuits (MMIC), is becoming prevalent. However, the use of such circuits presents difficulties associated therewith. A typical problem facing equipment designers and manufactures is determining the effect of actually deploying a MMIC in a circuit on the performance of the MMIC. Specifically, the presence of leads, bond wires, and grounding pins all affect the MMIC behavior.
Moreover, one installation of a MMIC involves the holding in place of a MMIC component in a circuit using a bonding substance such as an epoxy. However, a silver filled epoxy used for such purposes is prone to oxidation at any exposed points (Agxe2x89xa1 greater than Ag2O3) Oxidation of the epoxy has been found to cause the performance of the MMIC to appreciably change over time, e.g., as the epoxy oxidizes the impedance of the MMIC device changes.
Accordingly, one solution is to incarcerate the MMIC device in a protective container when deployed in a circuit in order to protect the device from degraded performance, such as due to the aforementioned oxidation of the bonding agent. For example, a container constructed using a top, such as a ceramic lid having a cavity disposed therein to envelope the MMIC, and the circuit board or ground plane as a base may be used. However, the use of such a container adds size as well as expense to the MMIC circuit and, therefore, may not be an acceptable solution in many commercial situations.
A relatively simple and inexpensive alternative to the above described container is to seal the MMIC in a resin, such as a polymeric resin, e.g., epoxy resin, pejoratively referred to as xe2x80x9cGLOBxe2x80x9d or xe2x80x9cGLOB TOPxe2x80x9d because of its seemingly arbitrary dollop application. This solution has the advantage of utilizing commonly available items, such as epoxy resin, being relatively easy to apply, and providing a seal suitable for preventing undesired oxidation of the bonding agent.
The heretofore practice of applying protective resins has been to both cover the bonding agent, such as the aforementioned silver filled epoxy, and the MMIC, often also including leads, bond wires, et cetera, i.e., applying a xe2x80x9cdollopxe2x80x9d of epoxy. However, the presence of the protective resin itself has been found to affect the performance characteristics of the MMIC. Specifically, the application of an epoxy resin completely incarcerating a MMIC, although providing protection of the MMIC device and connections thereto, affects the impedance of the device and, thus, can cause appreciable mismatches between the device and additional circuitry coupled thereto.
The performance characteristic changes due to the application of the protective resin are often acceptable at lower frequencies, such as 12-18 GHz. However, at the higher frequencies of millimeter wave technology, such as 20-100 GHz, the changes due to the application of the protective resin often become intolerable.
Accordingly, a need exists in the art for a system and method for deploying MMIC devices in circuitry wherein performance characteristics are predictable and maintainable over a reasonable operating life of the device.
A further need exists in the art for systems and methods for providing and maintaining desired performance characteristics of MMIC devices inexpensively. Accordingly, a need exists for techniques adapted for simple, preferably automated, deploying of MMIC devices. Likewise, a need exists for deployed MMIC devices to maintain a relatively small package size.
These and other objects, features and technical advantages are achieved by a system and method which utilizes a precisely controlled application of a protective substance where a MMIC, or other device susceptible to alteration of performance characteristics due to deploying and/or aging, is deployed in a circuit. According to a preferred embodiment of the present invention, an amount of protective substance is deposited in such a way as to completely seal areas subject to causing performance characteristic change/degradation while minimizing contact with the MMIC or other deployed device. For example, a most preferred embodiment of the present invention utilizes a MMIC device disposed on a circuit support structure surface, such as pedestal of an underlying ground plane, using a silver filled epoxy, wherein a controlled amount of protective epoxy is deposited so as to completely protect any exposed surfaces of the silver filled epoxy while only minimally contacting the MMIC device.
A technical advantage is achieved in that the performance characteristics of the MMIC device remain substantially unchanged by the application of the protective element of the present invention. Moreover, the performance characteristics of the MMIC device may be maintained throughout an extended operational life because elements, such as the above described silver based epoxy bonding agent, are protected from degradation which may result in a change in the performance characteristics of the MMIC device.
A still further advantage is achieved by the present invention as both the cost of the protective material and its application according to the present invention are relatively inexpensive and well suited for automated application.
A yet further advantage is realized according to the present invention because, although providing the desired protective characteristics, the size and bulk of the MMIC device in the finished circuit remains substantially unchanged, therefore avoiding increases in size of the final equipment incorporating the MMIC device.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.